The role of nitric oxide (NO) in the pathogenesis of chronic idiopathic colitis was assessed in juvenile rhesus macaques. The site, enzyme source and magnitude of no production was determined in juvenile rhesus diagnosed with chronic colitis/diarrhea, and scheduled for euthanasia. No production was evaluated systemically from plasma and urine levels of reactive nitrogen intermediates, and locally in the colon by NADPH diaphorase histochemistry. Inducible no synthase gene expression was assessed by reverse-transcriptase polymerase chain reaction (RT-PCR). Colitis animals showed elevations of plasma and urine levels of reactive nitrogen intermediates of 13 and 5 fold respectively, over controls. Nadph diaphorase activity was limited to the myenteric plexus in normal controls, while colitis animals showed reactivity in crypt abscesses, superficial epithelium and mucosal bands. Gene expression for inducible no synthase was found only in colitis specimens. These observations of increased no production, inducible no synthase activity and gene expression, and localization of no synthesis to sites of injury is consistent with the hypothesis that excess no contributes to the mucosal inflammation and clinical signs of this idiopathic colitis model system.